Data transmission at high speeds is difficult to accomplish at practical cost in a transmission medium that is subject to fast fading such as is found in cellular mobile radiotelephone systems. For example, in a system operating in the range of 850 megahertz, individual fades to 10 dB below the local mean power level in the vicinity of the receiver occur on the average at about thirty fades per second at a vehicle speed of about 35 MPH. In such an environment, information signal bits (data or digital voice) are typically of much shorter duration than the duration of a typical fade. A very poor error rate results. Errors result because information in the detected signal is either lost for the fade duration or it is accompanied by bursts of impairments which affect the ability of the receiver to identify properly the detected signal. Typical solutions have included approaches such as sending at a sufficiently low bit rate, e.g., tens of bits/second, to reduce the impact on error rate or such as sending multiple repetitions of every bit. Either solution greatly limits the effective bit rate of useful throughput of data, including digitized speech.
Chirp processing, or modulation, has been used for various purposes over the years at least since its use in radar systems as taught by J. R. Klauder et al. in "The Theory and Design of Chirp Radars" in The Bell System Technical Journal, Vol. 39, No. 4, pages 745-808, July 1960. In a number of applications of chirp processing, the bandwidth of energy in a pulse is expanded and time dispersed within the pulse; and the expanded bandwidth pulses are separately transmitted. Regulatory constraints prevent such bandwidth expansion in cellular radiotelephone systems.
Application of chirp to data transmission by using chirp to increase pulse bandwidth and then time compressing the chirped pulse was proposed by G. F. Gott in "H.F. Data Transmission Using Chirp Signals," Vol. 118, No. 9, pages 1162-1166 September 1971. As noted above, regulatory constraints make bandwidth expansion an unacceptable alternative for many applications.
A P. K. Lee et al. paper "Digital Chirp Modulation: Opportunities For Lower Cost And Higher Performance In Mobile-Satellite Communications," in Proceedings of the AIAA Institute of Electronics and Communications Engineers AIAA 8th Communications Satellite Systems Conference, 1980 at pages 696-702 shows bandwidth expansion employed for the relatively slow fading of a satellite system employing surface acoustic wave chirp filters to achieve large time-bandwidth products.
A K. Fong U.S. Pat. No. 3,484,6g3 shows a frequency shifted, sliding tone, sampled data system for satellite communication in which an improved signal-to-noise ratio is realized by increasing the bandwidth occupancy of the signal. Chirp signals, which are time spaced in their respective channels, span only one pre-chirp input pulse interval. As between the channels, the signals overlap in time by half an input pulse interval and are separately shifted in frequency in different ways by amounts determined by input baseband sample amplitudes. The chirp signals so modulated are then combined into a single channel.